This application relates generally to optical communications systems and methods for wavelength division multiplexed (WDM) optical networks, and more particularly to wavelength selective switch systems and methods having optimized optical performance for switching and managing the power of individual spectral channels of a multi-channel optical signal.
Multi-channel optical signals of the type to which the invention pertains comprise a plurality of spectral channels, each having a distinct center wavelength and an associated bandwidth. The center wavelengths of adjacent channels are spaced at a predetermined wavelength or frequency interval, and the plurality of spectral channels may be wavelength division multiplexed to form a composite multi-channel signal of the optical network. Each spectral channel is capable of carrying separate and independent information. At various locations, or nodes, in the optical network, one or more spectral channels may be dropped from or added to the composite multi-channel optical signal, as by using, for example, a reconfigurable optical add-drop multiplexer (ROADM). Reconfigurable optical add-drop multiplexer architectures are disclosed in commonly assigned U.S. Pat. Nos. 6,549,699, 6,625,346, 6,661,948, 6,687,431, and 6,760,511, the disclosures of which are incorporated by reference herein.
All optical switching nodes may comprise one or more wavelength selective switches (WSS) configured as ADD and/or DROP modules. The referenced patents disclose wavelength selective switch apparatus and methods comprising an array of fiber coupled collimators that serve as input and output ports for optical signals, a wavelength-separator such as a diffraction grating, a beam-focuser, and an array of channel micromirrors, one micromirror for each spectral channel. In operation, a composite multi-wavelength optical signal (also referred to herein as a “multi-channel optical signal”) from an input port is supplied to the wavelength separator. The wavelength separator spatially separates or demultiplexes the free-space multi-wavelength optical signal into an angular spectrum of constituent spectral channels, and the beam-focuser focuses the spectral channels onto corresponding ones of the channel micromirrors. The channel micromirrors are positioned such that each channel micromirror receives an assigned one of the separated spectral channel beams. The micromirrors are individually controllable and continuously pivotal (or rotatable) so as to reflect the spectral channel beams into selected output ports. This enables each channel micromirror to direct its corresponding spectral channel into any possible output ports and thereby switch the spectral channel to any desired output port. Each output port may receive none, one, or more than one of the reflected and so directed spectral channels. Spectral channels may be selectively dropped from a multi-channel signal by switching the channels to different output ports, and new input channels may be selectively added or combined with the original channels to form different multi-wavelength composite signals.
Several types of free-space beam separators are commercially available including but not limited to; reflection gratings such as high spatial frequency gratings holographically formed, low spatial frequency gratings such as an Echelle grating, which is a ruled grating, and transmission gratings which can be holographically formed in various polymers. Diffraction gratings used for wavelength selective separation WSS devices may also be polarization sensitive, having higher diffraction efficiency for one polarization state. Accordingly, to maintain an acceptable system insertion loss and polarization dependent loss, it may be necessary to employ a polarization diversity strategy where the polarization state of an input beam is decomposed into its orthogonal components, and the lower efficiency polarization component is rotated into the most efficient polarization state for diffraction. This requires that the two beam components be managed throughout the optics of the system, which increases the form factor of the device.
In wavelength selective switch architectures of the type to which the invention pertains, it is desirable to optimize the optical and mechanical characteristics of the system while providing a small form factor. Typically, in known apparatus and methods, although the input beam to the wavelength separator may be collimated and circular in cross-section, some wavelengths of the diffracted beam may be elliptical in cross-section and expanded due to anamorphic expansion of the beam caused by the diffraction grating. With other wavelengths, there may be no expansion, and with still others there could be compression, depending on the type of grating utilized and incident and diffracted angles. Also, the beam size at a channel micromirror is determined by the relationship between the input beam size, the anamorphic expansion or compression of the beam, and the focal length of the focusing optics. Therefore, in some wavelength switches that lack control of the beam size before diffraction and incident to the focusing optics, the spot size on a channel micromirror may not be readily controllable. This makes it necessary to size the micromirror sufficiently to accommodate possible variations in the input beam conditions and the viable focal length of the focusing optics. For instance, simply accepting the anamorphic beam expansion created by a grating may be insufficient to minimize the spot size on the channel micromirrors to create a high passband. In general, these factors prevent optimization of key optical parameters of the system, such as passband, and key mechanical parameters of the micromirrors, such as resonant frequency, which is inversely proportional to the mass of the mirror, and aerodynamic interaction between micromirrors, which is approximately proportional to their length to the third power. Thus, in some WSS designs known in the art, the input beam is modified by passing it through a prism pair anamorphic beam expander. This preferentially increases the beam expansion in one dimension. However, to substantially reduce the form factor, optical control of the beam sizes throughout propagation is necessary, and a one dimensional anamorphic beam expander is inadequate. Further, it can be advantageous to simultaneously (concomitant with the anamorphic beam expansion and beam size control) relay the angular rotation of a MEMS (micro electro mechanical systems) port mirror, and optically convert this angular rotation into a translation of the channels at the channel mirror to maintain channel frequency alignment as defined by the International Telecommunication Union, (ITU alignment). The aforementioned anamorphic beam expansion from the grating or from a prism pair alone does not accomplish this.
It is also desirable, for a number of reasons, to be able to monitor and control the power in individual spectral channels of the multi-wavelength optical signal. This includes the ability to completely block the power contained in a particular spectral channel. One reason for blocking the power in a channel is to afford “hitless” switching to minimize undesired crosstalk during the repositioning of an input spectral channel beam from one output port to a different desired output port. During repositioning, the channel micromirror scans the input spectral channel beam across (i.e., “hits”) intermediate ports, which couples unwanted light into the intermediate ports, and causes crosstalk. Thus, it is desirable either to completely block or to substantially attenuate the power in the beam during scanning so that unwanted light coupling is avoided. Another use of monitoring and controlling the optical power of a channel is to afford attenuation of that channel to some predetermined level.
The above-mentioned U.S. patents disclose one approach to power management and hitless switching that employs a spatial light modulator, such as a liquid crystal pixel array, to attenuate or completely block the power contained in the spectral channels. Each pixel in the liquid crystal array is associated with one of the spectral channels, and a separate focal plane is created at the location of the liquid crystal array such that a spectral spot corresponding to each channel is located on its associated pixel. Since the voltage applied to the pixel controls the light transmissivity of a pixel, the pixel can be made less transmissive or opaque to the transmission of light by applying an appropriate voltage, thereby attenuating or completely blocking the power in the spectral channel passing through that pixel. However, this approach has the disadvantage of requiring additional components, including a relay lens system to create a focal plane at the liquid crystal array, the liquid crystal array itself, and electronics to control the liquid crystal array. In addition to the added costs for such additional components, more physical space is needed to accommodate these components, which increases the overall size and complexity of the system.
U.S. Pat. No. 6,549,699 discloses another approach to power management of spectral channels in which the rotation of a channel micromirror about its switching axis (the axis parallel to the array of channel micromirrors) is controlled to vary the spatial location of the reflected spectral channel beam relative to its intended output port. Since the amount of power in a spectral channel that is coupled to an output port is a function of the coupling efficiency, a desired power level can be obtained by pivoting the channel micromirror a predetermined angle to decouple the optical beam relative to the output port to attenuate it by an amount corresponding to the desired output power level.
A disadvantage of this latter approach is that decoupling the spectral channel beam spatially repositions the beam along the switching axis. Depending upon the physical spacing of adjacent output ports, a portion of the beam may be cross-coupled into an adjacent output port, causing detrimental cross-talk between the ports. Increasing the physical spacing of the ports to decrease the cross-coupling undesirably increases the physical size of the device. Furthermore, as will be described in detail later, using this approach it is difficult to accurately control the power output levels of spectral channels due to the sensitivity of the coupling to rotation of the channel mirror about the switching axis.
It is desirable to have the following functions integrated into one wavelength selective switch system including: demultiplexing, multiplexing, fully flexible wavelength switching, non-blocking hitless switching, dynamic channel equalization to an arbitrary profile, variable optical attenuation, channel power monitoring, wavelength blocker, and wavelength connectivity confirmation. In addition, it is desirable for the architecture to achieve accurate attenuation of spectral channels, that the system be capable of operation as either an ADD or DROP module, and have a small form factor and low cost. Finally, all these functions should exist in a system with low polarization dependent loss, low insertion loss, and high optical passband while simultaneously maintaining accurate ITU channel alignment. While each of the aforementioned patents and embodiments known in the art addresses some of the integrated functionality desired in a wavelength selective switch, they fail to achieve a satisfactory level of integration of the desired functionality, performance, or accuracy of control. Therefore, it is desirable to provide compact, more flexible and more cost-effective architectures for achieving the multi-functionality of a wavelength selective switch, which includes achieving the aforementioned properties and functionality. It is to these ends that the present invention is directed.